1. Field of the Invention
The present invention relates to an adsorbent for hydrocarbons (which will be referred to as HC hereinafter) in an exhaust gas, and particularly, to an HC adsorbent made using zeolite and a process for producing the same.
2. Description of the Related Art
There is a conventionally known exhaust emission control device disposed in an exhaust system in a motor vehicle, e.g., an exhaust emission control device using a mixture of an HC adsorbent and a catalyst, and an exhaust emission control device using an HC adsorbent layer and a catalyst layer laminated one on another. In this case, zeolite is used as the HC adsorbent. Such exhaust emission control device is aimed at adsorbing by a zeolite HC that is contained at a high concentration in a low-temperature exhaust gas discharged immediately after starting of an engine, and desorbing HC from the zeolite in accordance with a rise in temperature of the exhaust gas to convert HC by the catalyst which is in an activated state.
To reliably achieve this aim, it is required that the temperature of desorption of HC from the zeolite be equal to or higher than a catalyst activating temperature. However, the following problem is encountered: The temperature of desorption of HC from zeolite commonly used at the present time is lower than the catalyst activating temperature and as a result, HC adsorbed cannot be converted sufficiently.
Accordingly, it is an objective of the present invention to provide an HC adsorbent of the above-described type, wherein HC desorbed from zeolite can be inhibited from flowing back into an exhaust gas, thereby providing an effect similar to that to be obtained in a case where the temperature of desorption of HC from the zeolite is higher.
To achieve the above objective, according to the present invention, there is provided an adsorbent for hydrocarbons (HC) in an exhaust gas, comprising an agglomerate of double-structure particles, each of which includes an HC-adsorbing zeolite core, and a ceramic coat wrapping the zeolite core and having a plurality of through-pores communicating with a plurality of pores in the zeolite core, wherein each of the double-structure particles is at least one of a double-structure particle including the zeolite core comprising a single zeolite particle, and a double-structure particle including the zeolite core comprising a plurality of zeolite particles, and each of the plurality of through-pores in the ceramic coat has such a shape that the HC is easy to flow into the through-pore and difficult to flow out of the through-pore.
When the adsorbent is formed as described above, HC contained at a high concentration in a low-temperature exhaust gas discharged immediately after starting of an engine is passed through the plurality of through-pores in the ceramic core and adsorbed in the plurality of pores in the zeolite coat. The HC is desorbed out of the pores in the zeolite core with rising of the temperature of the exhaust gas, but the desorbed HC is inhibited from flowing back into the exhaust gas by the ceramic coat. Therefore, the temperature at the end of the flowing-out of the HC is shifted to a level about 70xc2x0 C. higher than that in an adsorbent having no ceramic coat. Thus, the HC conversion rate can be increased largely.
Another objective of the present invention is to provide a process for producing an HC adsorbent of the above-described type, wherein the HC adsorbent can be produced easily.
To achieve the above objective, according to the present invention, there is provided a process for producing an adsorbent for hydrocarbons (HC) in an exhaust gas, the adsorbent comprising an agglomerate of double-structure particles, each of which includes an HC-adsorbing zeolite core, and a ceramic coat wrapping the zeolite core and having a plurality of through-pores, the process comprising the steps of:
preparing a liquid mixture of an agglomerate of zeolite particles and a ceramic coat-forming precursor solution, thereby forming at least one of a plurality of liquid drops each comprising a single of the zeolite particle wrapped with the precursor solution and a plurality of liquid drops each comprising a plurality of the zeolite particles wrapped with the precursor solution,
precipitating the precursor by drying the liquid drops, and trapping at least one of a liquid and vapor into pores in the zeolite particles,
forming a ceramic layer by pyrolysis of the precursor, and
creating the through-pores by escaping of the trapped vapor through the ceramic layer,
wherein the steps are carried out in the mentioned order.
If the above procedure is employed, the HC adsorbent can be produced easily and reliably.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.